This invention relates to an infrared sensitive composition and to a positive-working element using such a composition. More particularly, this invention relates to positive-working lithographic printing plates.
Lithographic printing depends upon the immiscibility of oily ink and water with the ink being retained on the oleophilic image areas of the plate and the water being retained on the hydrophilic non-image areas. Most lithographic printing plates have a radiation-sensitive coating applied to an aluminum printing plate substrate. These coatings can be selected to be sensitive to a wide range of radiations including ultraviolet radiation, visible radiation and infrared radiation. The coating may react to the radiation such that the portions of the coating exposed become soluble and are removed during development, thus becoming hydrophilic non-image areas. Such plates are known as positive-working printing plates. On the other hand, the coating may react to the radiation such that the exposed areas become hardened and insoluble and remain during development, becoming the oleophilic image areas. Such plates are known as negative-working printing plates.
One prior art lithographic plate of the negative-working type is described in U.S. Pat. No. 5,705,308. The coating is an infrared imaging composition containing a phenolic resin that is either mixed with or reacted with an o-diazonaphthoquinone derivative and which also contains a specific infrared radiation absorber.
Another prior art lithographic plate which is related but which is of the positive-working type is described in U.S. Pat. No. 5,858,626. The coating is also an infrared imaging composition containing a phenolic resin that is either mixed with or reacted with an o-diazonaphthoquinone derivative and which also contains an infrared radiation absorber. In this patent, the dry weight ratio of o-diazonaphthoquinone moiety to the infrared absorber in the coating composition is quite high and the diazonaphthoquinone moiety is at least 5 weight percent and more preferably 15 to 40 weight percent of the coating.
In the PCT International Application Publication Number WO 96/20429, another coating is described which is a naphthoquinone diazide ester of a phenolic resin or a naphthoquinone diazide ester mixed with a phenolic resin containing an infrared absorber. The plate is blanket (overall) exposed to ultraviolet radiation which solubilizes the entire coating as with a positive-working plate. The plate is then laser imaged to insolubilize the coating in the image areas as with a negative-working plate.
In the PCT International Application Publication Number WO 97/39894, the coating is an alkali soluble phenolic resin containing a compound which complexes with the resin and reduces the solubility. The coating also contains an infrared absorber which may also be the complexing compound. When exposed, the heating breaks down the complex so that the exposed areas become soluble. The complexing compound increases the solubility differential between the exposed and unexposed areas. The solubility of the coating is not affected by ultraviolet radiation.
Further prior art is disclosed in U.S. Pat. No. 5,962,192 which discloses a positive-working plate having a polymeric resin coating containing an organo azide compound and an infrared radiation sensitive dye. In this patent, the suitable resins are polyvinyl formal, acrylates, methacrylates and styrene. The radiation is absorbed by the dye to raise the temperature above the decomposition temperature of the organo azide. The plate is then blanket exposed to ultraviolet radiation which hardens the coating in the areas which were not exposed to the infrared radiation. This blanket exposure is necessary to increase the solubility contrast between the infrared exposed and non-exposed areas to a practical, workable level. The infrared exposed, non-image areas of the coating are then dissolved away. Alternately, the coating can first be blanket exposed to cross link and harden the coating. The coating is then infrared exposed in the non-image areas to solubilize the coating in those areas. An acidic organic solvent is used in the development process.